Characterization of autonomic receptors in the rat sublingual gland by biochemical and radioligand assays

Summary

The autonomic receptors of the rat sublingual gland were characterized by radioligand binding and by specific functional responses involving the release of K⁺ and the generation of cyclic AMP in vitro. Both muscarinic cholinergic and alpha₂-adrenergic receptors were present in moderately high density in the sublingual gland, as judged by the binding of the specific radioligands ³H-QNB and ³H-clonidine (B _max in pmol/g tissue=18.4±2.4 and 9.9±1.3, respectively). By contrast, although alpha₁ and beta-adrenergic receptors were detected, they were not present in large numbers. The B _max (pmol/g tissue) for the binding of ³H-prazosin and ³H-DHA were, respectively, 3.2±0.6 and 3.6±0.4. Stimulation of muscarinic cholinergic receptors with carbamylcholine (2×10⁻⁵ M) caused a net release of K⁺ from sublingual slices incubated in an enriched, oxygenated Krebs Ringer bicarbonate medium of 35±7% after 10 min of incubation. Norepinephrine, phenylephrine, clonidine and isoproterenol did not induced K⁺ release from the slice preparation, but actually reduced the basal or unstimulated release of K⁺. As in the submandibular and parotid glands, the release of K⁺ from sublingual slices had two components, a passive efflux and an active uptake which depended on the activation of an ouabain-sensitive Na⁺, K⁺ ATPase. The release of K⁺ was also dependent on the presence of Ca²⁺ (2.7 mM) in the incubation medium. Stimulation of beta-adrenergic receptors with isoproterenol (10⁻⁵ M) caused a 9-fold increase in the content of glandular cyclic AMP (from 4.0±0.6 to 36.0±2 pmol/mg/10 min). A similar increase was observed with norepinephrine (10⁻⁴ M) in the presence of phentolamine (10⁻⁴ M). Clonidine at concentrations of 10⁻⁵ and 10⁻⁴ M reduced the cyclic AMP content to below basal levels. The rat sublingual gland has functional cholinergic receptors and can release K⁺ in vitro in the presence of cholinergic agents. In contrast to the other major salivary glands, it has a low density of alpha₁-adrenergic receptors and fails to release K⁺ upon stimulation with alpha₁-agonists. The functional significance of a low density of beta-adrenergic receptors is still unclear, although there is a definite glandular cyclic AMP response upon stimulation with isoproterenol. Abundant alpha₂-adrenergic receptors in the sublingual gland are apparently negatively coupled to adenylate cyclase.